The present invention relates to improvements in methods and apparatus for dispensing fresh toner to an image development station in an electrostatographic copier/printer or the like. More particularly, it relates to apparatus for maintaining toner in a fluidized state to facilitate toner dispensing from relatively large storage containers.
In electrostatographic copiers and printers, pigmented thermoplastic particles, commonly known as xe2x80x9ctoner,xe2x80x9d are applied to latent electrostatic images to render such images visible. Often, the toner particles are mixed with and carried by somewhat larger particles of magnetic material. During the mixing process, the magnetic carrier particles serve to triboelectrically charge the toner particles to a polarity opposite that of the latent charge image. In use, the development mix is advanced, typically by magnetic forces, from a sump to a position in which it contacts the latent charge image. The relatively strong electrostatic forces associated with the charge image operate to strip the toner from the carrier, causing the toner to remain with the charge image. Thus, it will be appreciated that, as multiple charge images are developed in this manner, toner particles are continuously depleted from the mix and a fresh supply of toner must be dispensed from time-to-time in order to maintain a desired image density. Usually, the fresh toner is supplied from a toner supply bottle mounted upside-down, i.e., with its mouth facing downward, at one end of the image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth, and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the toner-metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the development mix.
It is well known that toner is a powdery substance that exhibits a considerable degree of cohesiveness and, hence, relatively poor flowability. Since the force of gravity alone does not usually suffice in causing toner to flow smoothly from the mouth of an inverted toner bottle, other supplemental techniques have been used to xe2x80x9ccoaxxe2x80x9d the toner from the bottle. For example, flow additives, such as silica and the like, have been added to the mix to reduce the troublesome cohesive forces between toner particles. See, e.g., the disclosure of U.S. Pat. No. 5,260,159 ; in which a xe2x80x9cfluidizationxe2x80x9d agent is added to a developer mix in a development sump to assist the movement of developer therein. While beneficial to a more consistent flow of developer, such substances influence other performance attributes of the development process and their effectiveness is therefore constrained. Automatically operated stirring devices or augers mounted within a horizontally oriented toner container, and thumping or vibrating devices connected to such containers have also been used to urge toner from its rest position towards an outlet or exit port. Such mechanical techniques work well when the toner container is relatively small (e.g., 2 to 5 liters) and the height of the toner column above the exit port is relatively low (e.g., lower than about 15 cm.) so as to avoid gravity-assisted compaction of the toner which further compromises flowability. But, as the size of the toner bottle or container increases, e.g., to accommodate high speed and wide format printing in which toner is consumed at extraordinarily fast rates, the above-noted flow-enhancing techniques have been found to be inadequate. In such high toner-consumption situations, toner sumps of the order of tens of liters are desirable in order to eliminate the need for frequent toner bottle replacements. The weight of the toner in these large volume containers is too great for conventional rappers and vibrators to keep the toner flowing through the outlet, and most of these devices only exacerbate the toner packing problem.
In U.S. Pat. No. 5,570,170, there is disclosed an apparatus for dispensing single-component, electrically conductive magnetic toner particles from a pair of inverted toner bottles mounted above a conventional development station in an electrostatic printing apparatus. A screen positioned at the mouth of each bottle serves to prevent toner flow from the bottle whenever the toner is piled up atop the screen. The toner-dispensing apparatus includes a pair of gas-permeable, but toner-impermeable, tubes that extend upwardly, into each bottle, a distance of about 30-60% of the height of the bottles. On command, pressurized gas is introduced into the tubes. As the gas passes through the tubes and into the toner bottles, it acts to fluidize the toner in the bottle in the vicinity of the bottle""s outlet, thereby enabling the toner to flow smoothly through the screen mesh and into the development station of the printer, as needed. In effect, the screen acts as a gate to prevent toner flow into the development station until the toner above the screen is fluidized. A microprocessor controls the application of pressurized gas to each of the bottles, switching from one bottle to the other as one bottle empties. By using two bottles, the machine operator can replace an empty bottle without shutting down the machine.
While the apparatus disclosed in the above patent may be advantageous in some respects in selectively dispensing magnetic toner to an image-development station, it is disadvantageous in that it requires one or more sources of compressed gas in order to effect the necessary fluidization of the toner mass in order to achieve passage of the toner through the metering screen at the mouth of each toner bottle. Further, to prevent toner dust from being blown out of the development station during toner dispensing, a vacuum must be created in the mouth of the development station. This dusting problem can be especially problematic as the size of the toner bottle increases to accommodate high speed and large format printing.
In view of the foregoing discussion, an object of this invention is to provide an improved method and apparatus for dispensing toner from high tower sumps or hoppers of the type used in high speed and/or high volume printing applications.
In accordance with a preferred embodiment of the invention, there is provided a toner-dispensing apparatus comprising a relatively large (e.g. 25-50 liters), vertically oriented container adapted to receive a fresh supply of toner. The toner container is adapted to be permanently installed within the framework of an electrostatographic printer or the like, and it is shaped somewhat like a funnel, having a lower portion with walls that converge towards a relatively small toner-outlet port in the container""s lowest-most portion. Rotatably supported within the toner container is a vertically oriented mechanical agitator that, during rotation, is adapted to break-up any agglomerations or compactions of toner particles in the container and to lift the toner vertically upwards, allowing such particles to settle in the container under the force of gravity. By such an arrangement, the contained toner is maintained in a fluidized or aerated state while toner is dispensed through the toner outlet port. In accordance with a first preferred embodiment, the mechanical agitator includes an auger blade having a plurality of apertures formed therein. The auger blade has a helical configuration and is shaped to advance toner substantially vertically upwards as the blade rotates. Preferably, the apertures are equally spaced apart along the length of the blade and are of a size to enable the lifted toner to easily drop downwardly, through the blade openings in order to maintain the level of toner in the container substantially constant. The auger blade is configured so that its outer edges are closely spaced from the container wall over the effective portion of the container that holds toner. According to a preferred embodiment, the axis of rotation of the auger is slightly displaced from the container""s central axis to facilitate break-up of compacted toner particles and to avoid any tendency for a large xe2x80x9cslugxe2x80x9d of toner to be advanced by the auger rotation.
According to another preferred embodiment, the above-noted mechanical agitator takes the form of a rotatably mounted substantially vertically oriented shaft having a series of radially-extending propeller blades. The blades are shaped so that, during rotation of the shaft, the toner particles are driven upwardly within the container.
In accordance with another aspect of the invention, a method is provided for enhancing the flowability of toner particles through an exit port located in the base of a substantially vertically oriented toner storage hopper. Such method comprises the steps of fluidizing the toner mass above the port by mechanically agitating the mass with propellers or an auger that rotate(s) about a substantially vertical axis within the toner container.
An advantageous technical effect of the invention is that fresh toner can be dispensed with enhanced reliability from a relatively large storage container in which the toner, but for the invention, would most certainly compact from its weight and from internal machine vibrations, and thereby resist movement from the container""s outlet port.
The invention and its advantages will become better understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like reference characters denote like parts.